The present invention relates to a digital recording and playback method and system by means of which a maximum amount of information may be recorded in digital form on a recording medium, which information may be faithfully reproduced upon playback.
Analog signals first may be converted to digital form by use of an analog to digital converter, and then recorded. On playback, the digital signals are easily reproduced without error, and then converted to analog form for reproduction of the original analog signals. However, faithful recording and reproduction of many analog signals requires operation at an extremely high bit rate not readily attainable using conventional recording and playback equipment employing a conventional recording medium, such as recording tape, or the like.
Data reduction techniques for reducing the average bit rate of digitized data are known. Those include, for example, the use of compression-decompression filter combinations wherein a compression filter at the recording unit of the system makes periodic estimates of values of the digitized sample signals based on previous, or previous and subsequent values thereof. The difference between an estimated, or predicted, value and a true value of the waveform is calculated. A decompression filter in the playback unit is used to make an identical estimate and, by addition to previous values, find the true value of the waveform. In general, the dynamic range of the difference is smaller than the dynamic range of the original time waveform, so that a smaller average bit rate is required for recording. In addition, further reduction in the average bit rate by encoding of difference signals prior to recording thereof also is known. For example, the use of a truncated Huffman type encoding means for further reducing the average number of bits needed to be recorded is known. A method of reducing the average bit rate by use of a second-order digital compression filter followed by a Huffman encoder is disclosed in an article by U. E. Ruttiman and H. S. Pipberger entitled, "Compression of the ECG by prediction of Interpolation and Entropy Encoding", IEE Transactions on Biomedical Engineering, Vol. BME-26, No. 11, pp. 613-623, Nov. 1979. Also, the recording of encoded digitized ECG signals is disclosed in an article by K. L. Ripley and J. R. Cox, Jr. entitled, "A computer System for Capturing Transient Electrocardiographic Data", Proc. Comput. Cardiol., pp. 439-445, 1976. There, digitized ECG signals are second-differenced using computer software, the second-differenced values are Huffman encoded, and the serial bit stream from the Huffman encoder is stored on disc storage means. With such prior art arrangements employing fixed speed transport of the recording medium, the speed must be sufficient to record the peak bit rates.
Variable speed recording and playback means also are well known. However, they have not been used in conjunction with compression-decompression filter combinations and encoder-decoder combinations such as described above for maximizing the amount of data which may be stored on a recording medium.